The present invention relates to a line head in which a plurality of photo emitters are arrayed, and an image forming apparatus incorporating the line head.
There has been developed an image forming apparatus using, as an exposer, a line head provided with a large number of photo emitters arrayed. Japanese Patent Publication No. 6-64229A discloses a technique for arraying EL (electroluminescence) elements on an optical printer head and supplying gradation data to each of the EL elements. Moreover, Japanese Patent Publication No. 11-198433A discloses a technique for eliminating inequality in a light emitting characteristic in a primary scanning direction in a printer head in which a plurality of LED chips are arrayed.
As shown in FIG. 32, a large number of organic EL elements Ea are arranged on a line head 10 in a primary scanning direction so that a photo emitter array 1 is formed. First and second power lines 2, 3 formed by thin film wirings. A feeding point 6 is provided on a power supply (VDD) side and a feeding point 7 is provided on a ground (GND) side. “A” denotes an anode electrode of the organic EL element Ea, “K” denotes a cathode electrode thereof, and a lead portion is extended to each of the electrodes.
A drive transistor Tr2 which is formed on the same substrate as the organic EL element Ea. A drain D of the drive transistor Tr2 is connected to the power line 2. A source S of the drive transistor Tr2 is connected to the anode electrode A of the organic EL element Ea through a lead portion. A gate G of the drive transistor Tr2 is connected through a wiring Ga to a source of a control transistor Tr1 which is not shown.
As illustrated in a circuit diagram of FIG. 33, a signal line 4 of a gate and a signal line 5 of a drain are connected to the control transistor Tr1. Each organic EL element Ea arranged in the photo emitter array 1 is connected between the first power line 2 connected to the feeding point 6 on the power supply (VDD) side and the second power line 3 connected to the feeding point 7 on the ground (GND) side.
A photo emitter using the organic EL element is a current driving unit, and a current flowing to the power line (VDD side) on the drain side of the drive transistor Tr2 and the power line (GND side) on the cathode (cathode electrode) side of the photo emitter is increased or decreased depending on the degree of the light emission of the photo emitter. The first and second power lines are fabricated by a thin film wiring. Resistances at both ends of the power lines depend on the size of a printer head and are in order of several W to several tens W.
In a case where all of the photo emitters are activated, the current of the photo emitter is in order of at least ten mA and a voltage to be applied to the photo emitter reaches several tens mV to a hundred mV. In a case where an organic EL element is used as the photo emitter, a current is changed for a slight difference in an applied voltage, that is, the organic EL element has such a property that the amount of light emission is changed greatly.
FIG. 30 shows an example of the arrangement of a control circuit in a case where a photo emitter array is formed in a primary scanning direction as described in the above publications. In this case, feeding points 6a and 6b on the power supply side (VDD) are provided on both sides of the photo emitter array 1. Moreover, feeding points 7a and 7b on the ground side (GND) are also provided on both ends of the photo emitter array 1. The feeding points 6a and 6b on the power supply side are connected to an external power line 17a and the feeding points 7a and 7b on the ground side are connected to an external power line 17b. 
The anode electrode A of the organic EL element Ea is connected to the source of the drive transistor Tr2 through a lead portion 27. Moreover, the cathode electrode of the organic EL element Ea is connected to the second power line 3 through a lead portion 28. A control circuit 15 generates a control signal for driving the drive transistor Tr2 and a control transistor which are provided for each photo emitter arranged in the photo emitter array 1. Control signal lines 18a and 18b are connected to the control circuit 15. A moisture-proof plate 20 covers the photo emitter array 1 and the control circuit 15 which are mounted on a substrate 30.
As shown in FIG. 31, the luminance I is varied depending on the position Y in the primary scanning direction in a case where a plurality of organic EL elements (photo emitters) Ea are arrayed and each photo emitter Ea is connected between the first and second power lines 17a, 17b. 
The lifetime of the photo emitter is reduced with an increase in the luminance. For this reason, inequality is generated in the lifetime of the photo emitter. When the amount of light emission has inequality, moreover, quality of printing is deteriorated. In the example shown in FIG. 30, thus, there is a problem in that the lifetime of the photo emitter is varied and the quality of printing is deteriorated.
In some cases, the photo emitters are divided into several groups in order to correspond to the formation requirement of various images. Also in such a case, there is a problem in that inequality in the luminances is generated among the respective groups.
Japanese Patent Publication No. 11-274569A discloses an example in which LEDs are arrayed on a printer head and each of the LEDs is divided into m groups each corresponding to n of pixels which are mutually adjacent to each other. The LEDs of each group are collectively connected to a common connection line, so that m of connection lines are provided in total. Further, n of matrix connection lines are provided to selectively operate one of the LEDs in the respective groups. By such a structure, it is possible to increase the intensity of light emission of the LED per constant time.
The above configuration is designed to record character data and gradation data is not supplied to a photo emitter. For this reason, there is a problem in that the formation of an image is restricted and a gradation image cannot be obtained.